In industry, widespread use is made of systems in which one or more vibration sensors or vibration rate sensors are installed on the outside of the housing of the motor that is to be monitored. In this context, reference can be made to U.S. Pat. No. 5,629,870. Here, the signal of the sensor is evaluated by a separate electronic unit and the operator is provided with a signal that can associate the measured vibrations with the various causes of malfunction. This calls for quite complex circuitry and an unwanted susceptibility to malfunction. Such an arrangement is also known from U.S. Pat. No. 5,726,911.